Compressed data base for radar land mass simulator

ABSTRACT

A process for producing a compressed data base for a radar land mass simulator which is descriptive of a selected land mass. Topographic maps or other data sources are used to determine prominent cultural features and terrain features such as ridge and valley lines. These features are identified by a string of contiguous straight line segments. The end points of each line segment can be identified in terms of their x, y, and z coordinates relative to a chosen origin. Here, however, only the first point in a string is identified in the terms of full x, y, and 2 coordinates, with each subsequent point identified in terms of the increment ( Delta x, Delta y, and Delta z) from the preceding point.

United States Fatem Hearts et all.

COMPRESSED DATA BASE FOR RADAR LAND MASS SHMULATOR Inventors: Robert A.Heartz, Deland, Fla; Dan Cohen, Cambridge, Mass.

Assignee: General Electric Company, New

York, NY.

Filed: Aug. 3, 1972 AppL No.: 277,778

US. Cl. 35/10A, 343/5 EM lint. Cl. S0910 W00 Field of Search 35/10.4;343/5 EM References Cited UNITED STATES PATENTS Primary Examiner-MalcolmF. Hubler Attorney-Allen E. Amgott et al.

[57] ABSTRACT A process for producing a compressed data base for a radarland mass simulator which is descriptive of a selected land mass.Topographic maps or other data sources are used to determine prominentcultural features and terrain features such as ridge and valley lines.These features are identified by a string of contiguous straight linesegments. The end points of each line segment can be identified in termsof'their x, y, and z coordinates relative to a chosen origin. Here,however, only the first point in a string is identified in the terms offull x, y, and z coordinates, with each subsequent point identified interms of the increment (Ax, Ay, and Az) from the preceding point.

4 Claims, 4 Drawing Figures COMPRESSED DATA BASE FOR RADAR LAND MASSSIMULATOR BACKGROUND OF TI IE INVENTION This invention relates generallyto simulated radar displays and more particularly to a process forproducing a compressed data base for such a display.

Airborne radars such as the plan position indicator (PPI) provideessential information for the navigation of an aircraft as well as forother purposes such as target identification. A degree of skill isrequired, however, to correctly interpret the information presented onthe radar display. Rather than acquiring this skill through experienceby observation of radar displays during aircraft flights over variousterrains and under various weather conditions, systems have beendeveloped to simulate actual radar returns on a radar display. Such asystem is disclosed in US. Pat. No. 3,131,247 whereinphototransparencies representing the land mass elevation and otherphototransparencies representing the radar reflectivity characteristicsof selected areas are used with a flying spot scanner to produce a PPIrepresentation. With such a system, as the aircraft moves to a positionwhere its radar surveys an area outside that covered by onetransparency, another transparency must be substituted. Also, ifcultural changes such as new bridges, buildings, etc., occur, thetransparencies must be modified to reflect the true return. The need forhigher resolution than can be obtained with the use of transparenciesrepresents another limitation on the use of this type of system.

The inherent limitations of the foregoing systems have led to a searchfor an improved approach. One approach involves reducing the terrain todigital words which describe the latitude, longitude, elevation andradar reflectivity of points at regular spaced intervals. The digitaldata base thus encoded is then used to produce a simulated radar return.

The number of data points contained in the data base using thisstraightforward approach depends upon the resolution required for theradar. The resolution of the radar in turn depends upon the number ofresolution elements in each line or sweep, typically 1,000. If onehundred foot resolution is needed, every one hundred square footinterval will be a resolution element and must be encoded. Encoding thearea covered by the 48 conterminous states would involve more thanresolution elements. This not only represents a Herculean datamanagement task, but also requires a tremendous storage capability. Inaddition, a PPI with one thousand resolution elements per sweep and fourthousand sweeps per scan requires the handling of 4 X 10 resolutionelements for each scan. Clearly, then, a brute force approach whereevery data element is processed is hopelessly uneconomical for asimulated real time PPI display. On the other hand, if resolution issacrificed by using fewer resolution elements, the simulator will loseits realism.

A more economical approach has been described by R. A. Heartz and W. M.Bunker in a paper entitled Radar Display Simulation for Training whichwas presented on Apr. 2, I971, at the Southeastern Section Meeting ofthe Analog/Hybrid Computer Educational Society. This approach basicallyinvolves the encoding of prominent features. These features areidentified by the x, y, and z coordinates of the end points ofcontiguous straight line segments defining the features. Al-

though much data which is not essential to the production of a realisticdisplay is thereby eliminated, identification of each line segment inthis manner still requires a large storage.

SUMMARY OF THE INVENTION In a preferred form of the invention, atopographic map describing the land mass to be encoded is used to selectthe features to be encoded. Ridge lines are identified as a string ofcontiguous straight line segments in terms of the end points of thesegments. A start point, such as the beginning of a ridge line isidentified fully in terms of its x, y, and z coordinates. The nextpoint, and each succeeding point in the string, need only be identifiedby its increments (Ax, Ay, and Az) from the preceding point. Valleylines and significant slope changes are similarly encoded. Areas ofdifferent radar reflectivity are also encoded by lines together with thereflectivity to the right and the left of the lines.

The land mass as thus described can then be portrayed on a radar displaythrough computation based on what an actual radar return would be at thesupposed position.

' BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a radar sweep on apattern of lines descriptive of a land mass;

FIG. 2 illustrates the method of determining whether the radar sweepwill intersect a particular line segment in the data base;

FIG. 3 illustrates the method of computing the point of intersection ofthe radar sweep and a line segment; and

FIG. 4 is a block diagram of circuitry which may be used to carry outthe computations.

DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with theinvention, a topographic map or other data source describing the regionor land mass to be encoded is utilized to select features to be encoded.Ridge lines, which are a most significant terrain radar feature, areidentified as a string of contiguous straight line segments in terms ofthe end points of the segments. To minimize the number of data points tobe stored, a start point, such .as the beginning of a ridge line, isselected. The line connecting the start point to the next point is thendefined in terms of the next point. In a similar manner, the next linesegment is identified by the subsequent end point. Moreover, as a meansof minimizing the number of binary digits required to identify points,the start point only is identified fully in terms of its x, y, and zcoordinates. Start points are flagged and coded as two words in the database. In the processing that follows, a start point takes two processingcycles with the results accumulated so that full accuracy is achievedwithout increasing data base word size or processing hardware.Subsequent points are identified in terms of increments (Ax, Ay, and A2)from the preceding point. The x, y, and z coordinates are relative to achosen origin, which could be, for example, a particular latitude,longitude, and the elevation at that point.

In a similar manner, valley lines or significant slope changes areencoded. As these are of lesser radar significance, the error criteriacan be relaxed so that better data compression is achieved. Areas ofdifferent radar reflectivity, such as water/land or land/cityboundaries, are also encoded within selected error criteria by linesthat define the boundaries and the reflectivity to the right and to theleft of the lines.

In addition, certain discrete targets such as oil tanks, towers, ships,buildings, appear on the radar display as bright blobs. These blobs havefew distinguishable features, but must be positioned accurately on thesimulator radar display both with respect to terrain and each other.Cultural features such as these are encoded by their Ax, Ay, and A2coordinates, an intensity and a width. Long slender cultural featuressuch as roads, dikes, small streams, etc., which are too narrow to beencoded by boundary lines, are encoded as target lines. These areencoded by the Ax, Ay, and A2 of the end points, an intensity and awidth. Complex cultural objects can be encoded by combinations of lineand point source targets.

Thus, a compressed digital model that completely defines dimensions andradar reflectivity characteristics is developed and stored as a networkof lines and point source targets. Some lines are flagged as imaginarylines. These are used where necessary to go from one feature to anotherand are subsequently bypassed in the processing.

The data base as thus encoded may then be placed in an appropriatestorage medium such as magnetic tape or disk.

On a particular simulated flight or mission, the portion of the database covering the terrain to be traversed is transferred from magnetictape to a media such as a drum which will permit more rapid access. Thepertinent data can easily be identified in terms of x and y coordinates.

Referring to FIG. I, a string of line segments 1-10 is shownrepresenting, say a ridge line. The ridge terminates at point 7 and isconnected by an imaginary line to point 8. Points 8-10, 8 may define thetop of a hill, for example.

Although not part of this invention, some description of how the data isutilized is desirable to demonstrate that compression is not achieved atthe sacrifice of quality.

At any particular time the area surveyed by the scan of the radar willencompass only a small portion of the mission data on the drum. The scanarea depends on the range setting of the radar and the simulatedposition of the aircraft. Referring again to FIG. 1, the scan area 12 ofthe radar of an aircraft at position A is shown. Although the scan areais circular, the data base for the particular scan is more convenientlytaken as a square. In addition, scan square 14 may be made larger thanthe diameter of the scan to include data which will come into viewbecause of movement of the aircraft during the scan. It should beunderstood that FIG. 1 is simplified in that a scan might typicallyinclude a thousand or more line segments, and overlapping datadefinition regions.

Since selecting the scan data involves sorting out a relatively smallamount of data from the mass of mission data as much as 1.4 secondsmight be required. Because of the aircraft movement the resultingdisplay would indicate non-realtime data. To obviate this result thescan data can be selected based on the calculated aircraft position 1.5seconds ahead of its present position, based upon its course andvelocity.

Having selected the data to be included in a particular scan, a furtherselection can be made, if desired, to reduce the data to that containedin a segment of the scan. Each reduction of the data reduces the totalnumber of computations which must be made in a given time period toprocess the data. For the purposes of this description, the reduction ofdata to that in a segment will not be made.

The next step is a determination of the particular line segments anddiscrete targets which will be intersected by the radar sweep to begenerated. In FIG. 1, for example, sweep line 16 intersects linesegments (4,5) and (6,7). One way to determine which line segments,contained within the scan, will be intersected is to compute the lengthsof the normals from the end points of each line segment to the sweepline. If the algebraic length of the normals of both end points of thesame line segment have the same sign, it indicates these points are bothon the same side of the sweep and that no intersection has occurred. Onthe other hand, when the algebraic signs are different, the end pointsare on opposite sides of the sweep, and an intersection has occurredprovided the range to the intersection is positive.

The geometry for this computation is shown in FIG. 2 for a particularpoint (x,,, y,,). The normal distance d between point (x,,, y,,) andsweep 18, is:

(yn y) cos 9 n sin 0 When a determination has been made that anintersection exists it is necessary to identify its location. This canbe done in terms of the distance or ground range of the intersectionfrom the ground location of the aircraft.

From FIG. 2 also it is possible to determine the ground range, R to theintersection of the sweep and the normal from the point (x,,, y,,) tothe sweep.

FIG. 3 illustrates the case of an intersection of sweep 20 with a linesegment having end points 1 and 2. It will be assumed that point 1 isalso the first point in a string. The normal distances D, and D from thepoints to the sweep are computed as are the ranges R, and R Using thenormal distances D, and D it is possible to determine the intersectionlocation through an iterative process. The values of D, and D includingtheir signs, are added, and the sum divided by two. In FIG. 3, the firstiteration will yield a new point 22. Since the normal distance for point22 has the same sign as point I, point 1 is now discarded, and the nextiteration is made using points 22 and 2. Eventually point n is reachedwith D equal to zero (to the desired degree of accuracy). Associatedwith point :1 will be a particular elevation which could be obtained bya separate interpolation between the known elevations of points 1 and 2.An interpolater has been devised, however, which duplicates themanipulations performed with respect to the D values, so as to providesimultaneously the value of the range, R,, and the elevation, Z,,.

Referring to FIG. 4, the distance values D, and D which have beencalculated are loaded into distance registers 24 and 26 respectively.The letter designations A and B on these registers merely indicate thatthe information pertaining to the first point of a line segment will beput in the A registers, while information relating to the second pointwill be put in the B registers. The

algebraic sum of D and D is produced in adder 28, and transferred tonormal accumulator 30. Timing and controls circuit 32 then delivers ashift right signal over line 34 which moves the amount in normalaccumulator 30 one place to the right, thereby effecting the division bytwo. The signs of D and D are also loaded into registers 24 and 26, andare delivered to sign control circuit 36. The sign of the quotient innormal accululator 30 is also delivered sign control circuit 36 andenables one of the two outputs. If the quotient sign is the same as thesign of D a receive signal (LOAD A) is delivered to distance register 24which will then receive the quotient from normal accumulator 30. If thequotient sign agrees with that of D a receive signal (LOAD B) isdelivered to distance register 26.

Depending upon whether a LOAD A or LOAD B signal has been produced, oneof the distance registers 24 and 26 will be loaded with a new normaldistance (the normal distance to the sweep from what was the midpointbetween the two original points). The process previously described isthen repeated. Repetition is continued until the quotient in normalaccumulator 30 becomes zero. Since each division is performed by a shiftright, the size of normal accumulator 30 limits the number of iterationsto reach zero. A normal distance of zero, of course, represents thepoint on the sweep where it intersects the line segment between the twooriginal points. Recognize zero circuit 38 receives the outputs ofnormal accumulator 30, and upon receipt of a zero, signals timing andcontrols circuit 32. The normal distances for the end points of the nextline segment determined to intersect with the sweep would then be loadedinto distance registers 24 and 26.

At the same time the computations relating to the normal distances arebeing performed, parallel processing of range and elevation data of thesame point is carried out. It will be recalled in connection with FIG.2, that the ground range, R to the intersection of the sweep with thenormal to the sweep from a point can be calculated. In FIG. 4, theranges R and R to the intersections of the sweep with the normals D andD are loaded into range registers 40 and 42. Their sum is taken in adder44 and delivered to range accumulator 46. The shift right signaldelivered by timing and controls circuit 32 over line 34 is alsoreceived by range accumulator 46 causing a division by two. The receivesignal (LOAD A or LOAD B) prepared either range register 40 or 42 toload the average range contained in range accumulator 46. This range isthe distance to the intersection of the sweep with the normal to thesweep from the midpoint of the line segment between the two originalpoints.

As with the normal distance computations, the ground range computationsare performed in an iterative manner until the range to the intersectionof the sweep and the line segment between the two original points iscontained in range accumulator 46.

In addition to the ground range to the intersection of the sweep and theline segment, the elevation at the intersection is also required. Theelevations of the end points of the line segment are contained inmemory. These are loaded into elevation registers 48 and 50, and summedin adder 52. The sum is contained in elevation accumulator 54 untiltheishift right signal delivered over line 34 causes the division bytwo. As was the case with the distance and range results, the quantitycontained in elevation accumulator 54 will be entered into eitherelevation register 48 or 50, depending upon whether a LOAD A or a LOAD Bsignal has been produced by sign control circuit 36.

The process is iterated in parallel with the distance and rangeprocessing so that at the time the distance becomes zero, theinterpolated elevation of the intersection is contained in elevationaccumulator 54.

Line targets are handled in a similar fashion, except only the range,size, and intensity are required when it has been determined that thesweep will intersect the target.

Discrete point targets do not require the interpolation routinedescribed in the preceding paragraphs. A discrete target intersects thesweep when its computed distance from the sweep (d,,) is less than thecomputed range (R,,) times A0 where A0 is the angle between sweeps.

The elevation and reflectivity or intensity information of anintersection is then read into a digital memory using computed range asthe address. This is the sweep elevation and reflectivity profile whichcorresponds to the flying spot scanner data as read from thetransparency system described in U.S. Pat. No. 3,131,247.

What is claimed is:

l. A process for producing a compressed data base for a radar land masssimulator, descriptive of a selected land mass relative to a chosenorigin comprising:

identifying land mass features desired to be displayed by a string ofcontiguous straight line segments in terms of the end points of thesegments;

selecting a start point for each string of contiguous straight linesegments;

recording the x, y, and z coordinates of each start point relative tothe origin; and

recording the Ax, Ay, and A2 of each subsequent point from the precedingpoint.

2. A process in accordance with claim 1 further including:

recording with the Ax, Ay, and A2 of each subsequent point the radarreflectivity codes to the right and left of the line connecting thepoint to the preceding point.

3. A process in accordance with claim 1 further including:

recording the Ax, Ay, and Az of discrete radar targets as points in saidstrings of contiguous line segments.

4. A process in accordance with claim 3 further including:

recording with the Ax, Ay, and A2 of each discrete radar target its sizeand intensity.

1. A process for producing a compressed data base for a radar land masssimulator, descriptive of a selected land mass relative to a chosenorigin comprising: identifying land mass features desired to bedisplayed by a string of contiguous straight line segments in terms ofthe end points of the segments; selecting a start point for each stringof contiguous straight line segments; recording the x, y, and zcoordinates of each start point relative to the origin; and recordingthe Delta x, Delta y, and Delta z of each subsequent point from thepreceding point.
 2. A process in accordance with claim 1 furtherincluding: recording with the Delta x, Delta y, and Delta z of eachsubsequent point the radar reflectivity codes to the right and left ofthe line connecting the point to the preceding point.
 3. A process inaccordance with claim 1 further including: recording the Delta x, Deltay, and Delta z of discrete radar targets as points in said strings ofcontiguous line segments.
 4. A process in accordance with claim 3further including: recording with the Delta x, Delta y, and Delta z ofeach discrete radar target its size and intensity.